1. Field of the Invention
The present invention relates to a light-emitting material. The present invention relates to a light-emitting element having a pair of electrodes and a layer which includes a light-emitting substance that emits light by being applied with an electric field. The present invention relates to a light-emitting device having such a light-emitting element.
The present invention relates to a light-emitting device, a lighting device, and an electronic device each using the light-emitting element.
2. Description of the Related Art
An organic compound can take a wider variety of structures compared with an inorganic compound, and it is possible to synthesize a material having various functions by appropriate molecular-design of an organic compound. With those advantages, electronics utilizing a functional organic material has been attracting attention in recent years.
For example, a solar cell, a light-emitting element, an organic transistor, and the like are exemplified as electronic devices utilizing an organic compound as a functional material. These are devices taking advantage of electric properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been making remarkable development.
It is considered that the light emission mechanism of a light-emitting element is as follows: when voltage is applied between a pair of electrodes that interpose a light-emitting layer, electrons injected from a cathode and holes injected from an anode are recombined in an emission center of the light-emitting layer to form a molecular exciton, and energy is released to emit light when the molecular exciton relaxes to the ground state. Singlet excitation (S*) and triplet excitation (T*) are known as excited states. Light emission is considered possible through either singlet excitation or triplet excitation. In addition, the statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
At room temperature, a compound that is capable of converting a singlet excited state to luminescence (hereinafter, referred to as a fluorescent compound) exhibits only luminescence from the singlet excited state (fluorescence), not luminescence from the triplet excited state (phosphorescence). Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
On the other hand, by using a compound that converts a triplet excited state into luminescence (hereinafter referred to as a phosphorescent compound), the internal quantum efficiency can be improved from 75% to 100% theoretically. In other words, emission efficiency can be 3 to 4 times as much as that of the fluorescence compound. For these reasons, in order to achieve a highly-efficient light-emitting element, a light-emitting element using a phosphorescent compound has been actively developed recently.
When a light-emitting layer of a light-emitting element is formed using the above-described phosphorescent compound, in order to suppress concentration quenching of the phosphorescent compound or quenching due to triplet-triplet annihilation (T-T annihilation), the light-emitting layer is often formed so that the phosphorescent compound is dispersed in a matrix of another substance. In this case, the substance used to form the matrix is called a host material, and the substance dispersed throughout the matrix like the phosphorescent compound is called a guest material.
When a phosphorescent compound is used as a guest material, a host material is needed to have triplet excitation energy (the difference in energy between the ground state and the triplet excited state) higher than that of the phosphorescent compound. Therefore, a substance that has high triplet excitation energy has been developed.
For example, in Non Patent Document 1, a material which has a quaterphenylene skeleton is used as a host material of a phosphorescent compound which exhibits blue light emission and as a hole-transporting layer.
[Reference]
    [Non-Patent Document]    [Non Patent Document 1] J. Kido et. al., Chemistry Letters, Vol. 36, No. 2, pp. 316-317 (2007)